Abstract
To define the mechanics and energetics of the myosin motor action in muscles, it is mandatory to know fundamental parameters such as the stiffness and the force of the single myosin motor, and the fraction of motors attached during contraction. These parameters can be defined in situ using sarcomere−level mechanics in single muscle fibers under the assumption that the stiffness of a myosin dimer with both motors attached (as occurs in rigor, when all motors are attached) is twice that of a single motor (as occurs in the isometric contraction). We use a mechanical/structural model to identify the constraints that underpin the stiffness of the myosin dimer with both motors attached to actin. By comparing the results of the model with the data in the literature, we conclude that the two-fold axial stiffness of the dimers with both motors attached is justified by a stiffness of the myosin motor that is anisotropic and higher along the axis of the myofilaments. A lower azimuthal stiffness of the motor plays an important role in the complex architecture of the sarcomere by allowing the motors to attach to actin filaments at different azimuthal angles relative to the thick filament.
Highlights
The sarcomere is the structural unit of striated muscle
The arrays of 294 motors work cooperatively in each half−sarcomere, so that in order to define the molecular basis of muscle energetics and efficiency, it is necessary to know either the stiffness and the force of the attached myosin motor or the fraction of motors (f ) attached in an isometric contraction, and how this fraction depends on the load or shortening velocity
The most suitable preparation for measuring those parameters under physiological conditions is the single muscle cell, or fiber, in which the measurements can be made at the level of the half-sarcomere, with nm−μs resolution [2–5]. f can be determined by comparing the compliance of the half−sarcomere (Chs) during contraction and in rigor, the state reached after the depletion of ATP, with all 294 of the myosin motors in each half-thick filament attached to actin [4,6]
Summary
The sarcomere is the structural unit of striated muscle (skeletal and cardiac). In each sarcomere (~2–2.5 μm long), the contractile protein myosin and actin are organized in well-ordered and parallel arrays of filaments. The myosin motor ( called the myosin head) attaches to actin, forming a cross−bridge, and undergoes a structural working stroke that drives the thin filament towards the center of the sarcomere. The arrays of 294 motors work cooperatively in each half−sarcomere (hs, the functional unit of striated muscle), so that in order to define the molecular basis of muscle energetics and efficiency, it is necessary to know either the stiffness and the force of the attached myosin motor (cross−bridge) or the fraction of motors (f ) attached in an isometric contraction, and how this fraction depends on the load or shortening velocity (for an extensive reference see [1]). F can be determined by comparing the compliance of the half−sarcomere (Chs) during contraction and in rigor, the state reached after the depletion of ATP, with all 294 of the myosin motors in each half-thick filament attached to actin [4,6]. Εz with tan2(α) varying between 0 and ∞, and ∼ between ε and ∞
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